1. Field of the Invention
This invention relates to a process for the preparation of a porous membrane, and particularly to a process for making a porous membrane using waxy and/or fatty substances.
2. Prior Art
It has hitherto been known a process for the preparation of a porous membrane made of polyvinyl alcohol, wherein an aqueous solution of polyvinyl alcohol is prepared, then vinyl acetate is added to the aqueous solution, and said vinyl acetate is emulsion-polymerized using a redox polymerization initiator of tartaric acid-hydrogen peroxide system to prepare an aqueous dispersion of polyvinyl acetate, followed by drying to form a membrane and extracting and removing the polyvinyl acetate from said membrane using a solvent. In the membrane prepared by this known process, a number of void pores is formed by extracting with a solvent fine particles of the polyvinyl acetate dispersed uniformly in the polyvinyl alcohol. The membrane thus formed is an interesting and unique one in that the void pores thereof are communicated with each other and the porosity of the membrane reaches up to about 0.87, and it has been expected that the membrane must be of wide utility when used as filtering and absorbing materials, for example in the technical fields of foodstuff, medical treatments and processes for treating waste water.
However, this known process has the following disadvantages. It is inevitable to use a water-soluble high polymer, such as polyvinyl alchol, since a porous membrane is formed by the emulsion polymerization of vinyl acetate. As a result, the membrane made of a water-soluble high polymer is necessarily subjected to a treatment for allowing the same to be insoluble in water. A further disadvantage is that a particular polymerization initiator must be used under a particular emulsion polymerization condition to form a porous membrane, since vinyl acetate is used as the starting material. Accordingly, this known process is not always satisfactory in view of the producibility and economical standpoint because of the aforementioned limitations in production steps. A still further disadvantage of this known process resides in that the porosity of the resultant porous membrane is hardly to be controlled or modified freely, since the condition for the emulsion polymerization step included therein is limited as aforementioned.